The challenge to reduce the size of a machine and vibrations/noise generated in the machine is an essential object to be attained in the development of the machine, and developers of machines including hermetic compressors are naturally confronted with such a challenge.
In the development of an electric motor, reducing the overall height of a stator constituting the motor is a means for achieving reduction in size. In a stator with short-pitch concentrically distributed windings such as shown in FIG. 14, the height of coil ends is limited in such a manner that coils 20 are shaped in the axial direction after the coils 20 have been inserted in slots of a stator core 10.
Another example of size reduction means is a stator with concentrated windings. The use of this type of stator has been increased because there has also been a demand for increasing the efficiency of electric motors. In the case of this type stator, coils are directly wound around teeth with resin insulators for electrical insulation interposed therebetween and, therefore, the height of the coil ends can be effectively reduced. The use of electric motors with this type of stator in hermetic compressors and other machines has been rapidly increased in recent years.
Alternative refrigerants, which do not destroy the ozone layer, and which are R134a, etc., have been used in hermetic compressors in attempts to solve environmental problems in recent years. However, the electrical conductivity of R134a is higher than those of the conventional refrigerants, so that the leak current from the coil ends of the electric motor exposed to R134a is increased. Therefore, a brushless motor with concentrated windings having a reduced coil end height that is a reduced area of exposure to R134a, in particular, is used markedly effectively as the hermetic compressors using the refrigerant R134a.
A sinusoidal wave driving system is an effective means for reducing vibration and noise from an electric motor. Another vibration/noise reduction means is to use distributed windings capable of generating smoothly formed magnetic fields.
Also, there are many instances of use of full-pitch windings in power generators. For example, Japanese Unexamined Patent Publication No. 55-157948 discloses such windings.
Further, Japanese Unexamined Patent Publication No. 62-107660 discloses a technique related to a manufacturing method of the windings. This technique is used to shape coils after insertion of the coils so that the size of the coil end in the radial direction is substantially equal to the thickness of the wiring forming the coils, and the purpose of this technique is to effectively dissipate heat produced by a power generator (for example, the description in the publication, page 3, from line 15 in the upper right section to line 4 in the lower left section under the caption “Object of the Invention”) and, to achieve this object, the coil end inwardly bent in the radial direction of the stator (for example, the description in the publication, page 5, from line 6 to line 8 in the upper right section) is made straight by being shaped in the radial direction of the stator (for example, the description in the publication, page 5, at line 9 in the upper right section) by a manufacturing apparatus described with reference to “FIG. 10” in this publication.
Japanese Unexamined Patent Publication No. 2-221688 discloses a brushless motor having a stator with three-phase full-pitch windings in the three phases, in which coils are inserted in slots in a one coil per one slot manner, and an hermetic compressor including the motor (for example, the description in the publication, from line 14 in the lower right section of page 2 to line 3 in the upper left section of page 3, and “FIG. 1” or “FIG. 2 in this publication).
In a stator having a coil arrangement similar to that in the above-described stator having short-pitch or full-pitch concentrically distributed windings, i.e., a coil arrangement in which one coil is inserted in one slot and other coils are inserted in two adjacent slots, in other words, in a coil configuration in which, on the opposite sides of the coil end of one coil, the coil ends of other coils are concentrically arranged (regardless of whether each of these coils is of the same phase or not), the adjacent coil ends contact each other and change in shape of the coil ends in a direction along which the coils are arranged is limited, so that the height of the coil ends cannot be reduced.
Also, an electric motor having a stator with concentrated windings (hereinafter referred to as “concentrated-winding motor”) can be reduced in coil end height, but has a problem that vibrations are increased due to its winding configuration.
As means for solving this problem, a sinusoidal wave driving system may be used in place of a conventional rectangular wave driving system for a concentrated-winding motor. In the sinusoidal wave driving system, however, switching is performed with an increased frequency and the effect of reducing the leak current by using concentrated windings is reduced to half in an atmosphere such as of R134a which provides high electrical conductivity.
In an hermetic compressor having a motor driven by this system, therefore, it is necessary to maintain a sufficiently large insulation distance from a housing. Thereby, the reduction in size of the hermetic compressor is limited.
Now, it is apparent that if the coil ends of full-pitch winding are shaped only in the radial direction of the stator, the height of the coil ends in the axial direction is increased.